Up to now, a technique in which an acceleration sensor is buried in a back surface of a tire tread, and a road surface condition, for example, an asphalt road, a snowy road, or a frozen road, is estimated on the basis of a detection signal of the acceleration sensor has been proposed (for example, refer to Patent Literature 1). Specifically, in the case where the acceleration sensor is buried in the back surface of the tire tread, when a portion corresponding to a location in which the acceleration sensor is disposed in the tire tread contacts a road surface in association with the rotation of the tire, a vibration component corresponding to the road surface condition is superimposed on a detection signal of the acceleration sensor. For that reason, up to now, a frequency component of the vibration acquired when the tire portion corresponding to the portion in which the acceleration sensor is disposed in the tire tread contacts the road surface is analyzed to estimate the road surface condition.
However, when a tire wear occurs, the road surface condition cannot be accurately estimated by merely performing the frequency analysis of vibration applied to the tire. For example, a decrease in a friction coefficient between the tire and the road surface can be detected on the basis of an increase of high frequency component superimposed on the detection signal during a road contact duration. Herein, the road contact duration is a duration while a part of the tire is in contact with the road surface. The tire wear also causes an increase of the high frequency component. That is, both the tire wear and travelling on a low μ road surface may cause the increase of high frequency component. Herein, low μ road is a road whose road surface friction coefficient (hereinafter referred to as “μ”) is small. Thus, a factor causing the increase of high frequency component is difficult to be determined when a tire wear occurs.